A coordinate system is used to define a location on the Earth. It is created in association with a map projection, datum, and reference ellipsoid and describes locations in terms of distances or angles from a fixed reference point. The system may be either a Cartesian system, with coordinates based on orthogonal or 90-degree angles, or it may be polar, based on angles measured from a point such as the center of the Earth. For example, in the latitude/longitude system, positions are described based on angular measurements north or south of the equator and east or west of the Prime Meridian, which runs through Greenwich, England. This is considered a polar system [See Figure 1].

Coordinates are used to describe positions in two- or three-dimensional space and specify a particular location. A two-dimensional system uses a coordinate pair, typically referred to as X and Y, to describe a horizontal position on the Earth. A three-dimensional system adds the height or elevation to create an X, Y, Z position.

Figure 1: Example of a Polar System (SIC 2002).

Some coordinate systems extend over the entire globe, while others are used exclusively for specific regions of the Earth. Examples of global coordinate systems include Latitude/Longitude, Universal Transverse Mercator (UTM), World Geographic Reference System (WGRS), and various military grid reference systems. Examples of local coordinate systems include numerous national grid systems, as well as Universal Polar Stereographic, which is used for the polar regions of the globe.